Dissolved air flotation (DAF) systems are often used to separate particulate material and gases from solutions such as wastewater. The systems typically employ the general principle that bubbles rising through a solution attach to and carry away particles or gases suspended or dissolved in the solution. As bubbles reach the surface of the solution, the attached particles coalesce to form a froth or floc that is easily collected while the entrapped gases within the bubbles dissipate into the air.
Traditional DAF systems typically introduce small air bubbles into the lower portion of a relatively large tank filled with the solution to be treated. The air bubbles rise through the solution and attach to particles in the solution and gases dissolved in the solution transfer from the solution into the bubbles. The tank includes an outlet that directs purified liquid through the tank as effluent at a flow rate consistent with the inlet rate of the solution.
While traditional DAF systems work well for their intended applications, the processing time and particle/gas removal efficiency typically depends on the residence time of the bubbles in the solution. The residence time, in turn, relates to the bubble buoyancy, the depth of the bubbles within the solution, and the amount of turbulence in the solution. As a result, traditional DAF systems employ relatively large and costly tanks having correspondingly large "footprints". The footprints maximize the gas transfer time from the solution into the bubbles and the probability that particles will contact the bubbles during the residence time available within the tank. Moreover, the relatively large footprints also allow the bubbles sufficient time to float to the surface.
In an effort to reduce somewhat the tank size for a DAF system, one proposal disclosed in U.S. Pat. No. 4,022,696 employs a rotating carriage and floc scoop. The carriage directs an inlet solution substantially horizontally along a flow path to increase the path length for bubble travel, and correspondingly increasing the residence time. Unfortunately, while the tank size reduction is alleged as an advantage, the problem with performance tied to residence time still remains. This appears particularly true with the level of turbulence created by the rotating carriage and scoop.
Another proposal, disclosed in U.S. Pat. No. 5,538,631, seeks to address the turbulence problem by incorporating a plurality of spaced apart and vertically arrayed baffles. The baffles include respective vanes angularly disposed to re-direct the flow of liquid from an inlet positioned at the bottom of the tank. Liquid flowing through the tank deflects upwardly as it traverses the vanes, allegedly reducing the extent and intensity of turbulence generated near the inlet to the tank.
While this proposal is intended to reduce the turbulence problem relating to bubble residence time, the redirected fluid still appears to affect bubbles rising in other areas of the tank, and influencing the residence time of such bubbles. Moreover, the proposal fails to address the basic problem of DAF performance being dependent on bubble residence time.
In an effort to overcome the limitations in conventional DAF systems, those skilled in the art have devised air-sparged hydrocyclones (ASH) as a substitute for DAF systems. One form of air sparged hydrocyclone is disclosed by Miller in U.S. Pat. No. 4,279,743. The device typically utilizes a combination of centrifugal force and air sparging to remove particles from a fluid stream. The stream is fed under pressure into a cylindrical chamber having an inlet configured to direct the fluid stream into a generally spiral path along a porous wall. The angular momentum of the fluid generates a radially directed centrifugal force related to the fluid velocity and the radius of the circular path. The porous wall is contained within a gas plenum having gas pressurized to permeate the porous wall and overcome the opposing centrifugal force acting on the fluid.
In operation, the unit receives and discharges the rapidly circulating solution while the air penetrates the porous wall. Air bubbles that emit from the wall are sheared into the fluid stream by the rapidly moving fluid flow. Micro-bubbles formed from the shearing action combine with the particles or gases in the solution and float them toward the center of the cylinder as a froth in a vortex. The centrally located froth vortex is then captured and exited through a vortex finder disposed at the upper end of the cylinder while the remaining solution exits the bottom of the cylinder.
One variation in the general ASH construction, as described in U.S. Pat. Nos. 4,838,434 and 4,997,549, includes employing a froth pedestal at the bottom of the cylinder to assist directing the froth vortex through the vortex finder. Another ASH modification includes replacing the vortex finder and froth pedestal with a fixed splitter disposed at the bottom of the cylinder and having a cylindrical knife edge. The edge is positioned to split the helically flowing solution into components dependent upon the specific gravity of the components.
While the foregoing ASH constructions present significant advantages over conventional DAF systems by generating far more bubble-particle collisions and far more surface area for gas transfer to decrease the solution processing time, the separation capability of an ASH system by itself remains somewhat limited. This is because relatively large amounts of solution typically remain in the froth, and significant particle concentrations often remain in the solution. Additionally, the presence of the froth pedestal tends to compromise the uniformity of the helically flowing solution.
Therefore, the need exists for an economical flotation separation system capable of separating particulate matter and gases from a solution at relatively high throughput rates without the dependence on residence time. Moreover, the need exists for a flotation separation system of greatly reduced size to minimize costs and the space required to treat solutions. An additional need exists for a flotation separation system having a modularized capability for flexibly adapting to a variety of solution treatment environments and applications. The flotation separation system and method satisfies these needs.